Save editor scroll/pan position per sprite

[towerofnix]

Like Scratch 3.0 does now. It's a pain to have to scroll back to a specific position every time you select a sprite!

This isn't as big of a problem with the script browser, but it's still a little annoying.

To do this, we'd need to save the scroll position on each sprite - probably just a runtime attribute; serializing scroll position isn't a big deal and not worth the effort of making it backwards+forwards-compatible.

Saving would need to happen when a different sprite is selected, and when a script is selected, but NOT when a script is already selected.

Another idea would be to have the scroll position on each block be saved. That's probably a separate feature, though. (Note to self: also not super high priority if people separate their code into relatively small functions - apparently there was once a guideline "never make a function that's bigger than the viewport"! But still a handy feature.)